libmodbus/src/modbus-rtu.c

/*
 * Copyright © 2001-2010 Stéphane Raimbault <stephane.raimbault@gmail.com>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser Public License as published by
 * the Free Software Foundation; either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser Public License for more details.
 *
 * You should have received a copy of the GNU Lesser Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <fcntl.h>
#include <string.h>

#include "modbus.h"
#include "modbus-private.h"

#include "modbus-rtu.h"
#include "modbus-rtu-private.h"

/* Table of CRC values for high-order byte */
static const uint8_t table_crc_hi[] = {
    0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0,
    0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41,
    0x00, 0xC1, 0x81, 0x40, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0,
    0x80, 0x41, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40,
    0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1,
    0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0, 0x80, 0x41,
    0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1,
    0x81, 0x40, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41,
    0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0,
    0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x00, 0xC1, 0x81, 0x40,
    0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1,
    0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40,
    0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0,
    0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x00, 0xC1, 0x81, 0x40,
    0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0,
    0x80, 0x41, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40,
    0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0,
    0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41,
    0x00, 0xC1, 0x81, 0x40, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0,
    0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41,
    0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0,
    0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x00, 0xC1, 0x81, 0x40,
    0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1,
    0x81, 0x40, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41,
    0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0,
    0x80, 0x41, 0x00, 0xC1, 0x81, 0x40
};

/* Table of CRC values for low-order byte */
static const uint8_t table_crc_lo[] = {
    0x00, 0xC0, 0xC1, 0x01, 0xC3, 0x03, 0x02, 0xC2, 0xC6, 0x06,
    0x07, 0xC7, 0x05, 0xC5, 0xC4, 0x04, 0xCC, 0x0C, 0x0D, 0xCD,
    0x0F, 0xCF, 0xCE, 0x0E, 0x0A, 0xCA, 0xCB, 0x0B, 0xC9, 0x09,
    0x08, 0xC8, 0xD8, 0x18, 0x19, 0xD9, 0x1B, 0xDB, 0xDA, 0x1A,
    0x1E, 0xDE, 0xDF, 0x1F, 0xDD, 0x1D, 0x1C, 0xDC, 0x14, 0xD4,
    0xD5, 0x15, 0xD7, 0x17, 0x16, 0xD6, 0xD2, 0x12, 0x13, 0xD3,
    0x11, 0xD1, 0xD0, 0x10, 0xF0, 0x30, 0x31, 0xF1, 0x33, 0xF3,
    0xF2, 0x32, 0x36, 0xF6, 0xF7, 0x37, 0xF5, 0x35, 0x34, 0xF4,
    0x3C, 0xFC, 0xFD, 0x3D, 0xFF, 0x3F, 0x3E, 0xFE, 0xFA, 0x3A,
    0x3B, 0xFB, 0x39, 0xF9, 0xF8, 0x38, 0x28, 0xE8, 0xE9, 0x29,
    0xEB, 0x2B, 0x2A, 0xEA, 0xEE, 0x2E, 0x2F, 0xEF, 0x2D, 0xED,
    0xEC, 0x2C, 0xE4, 0x24, 0x25, 0xE5, 0x27, 0xE7, 0xE6, 0x26,
    0x22, 0xE2, 0xE3, 0x23, 0xE1, 0x21, 0x20, 0xE0, 0xA0, 0x60,
    0x61, 0xA1, 0x63, 0xA3, 0xA2, 0x62, 0x66, 0xA6, 0xA7, 0x67,
    0xA5, 0x65, 0x64, 0xA4, 0x6C, 0xAC, 0xAD, 0x6D, 0xAF, 0x6F,
    0x6E, 0xAE, 0xAA, 0x6A, 0x6B, 0xAB, 0x69, 0xA9, 0xA8, 0x68,
    0x78, 0xB8, 0xB9, 0x79, 0xBB, 0x7B, 0x7A, 0xBA, 0xBE, 0x7E,
    0x7F, 0xBF, 0x7D, 0xBD, 0xBC, 0x7C, 0xB4, 0x74, 0x75, 0xB5,
    0x77, 0xB7, 0xB6, 0x76, 0x72, 0xB2, 0xB3, 0x73, 0xB1, 0x71,
    0x70, 0xB0, 0x50, 0x90, 0x91, 0x51, 0x93, 0x53, 0x52, 0x92,
    0x96, 0x56, 0x57, 0x97, 0x55, 0x95, 0x94, 0x54, 0x9C, 0x5C,
    0x5D, 0x9D, 0x5F, 0x9F, 0x9E, 0x5E, 0x5A, 0x9A, 0x9B, 0x5B,
    0x99, 0x59, 0x58, 0x98, 0x88, 0x48, 0x49, 0x89, 0x4B, 0x8B,
    0x8A, 0x4A, 0x4E, 0x8E, 0x8F, 0x4F, 0x8D, 0x4D, 0x4C, 0x8C,
    0x44, 0x84, 0x85, 0x45, 0x87, 0x47, 0x46, 0x86, 0x82, 0x42,
    0x43, 0x83, 0x41, 0x81, 0x80, 0x40
};

static int _modbus_set_slave(modbus_t *ctx, int slave)
{
    if (slave >= 1 && slave <= 247) {
        ctx->slave = slave;
    } else {
        errno = EINVAL;
        return -1;
    }

    return 0;
}

/* Builds a RTU request header */
static int _modbus_rtu_build_request_basis(modbus_t *ctx, int function,
                                           int addr, int nb,
                                           uint8_t *req)
{
    req[0] = ctx->slave;
    req[1] = function;
    req[2] = addr >> 8;
    req[3] = addr & 0x00ff;
    req[4] = nb >> 8;
    req[5] = nb & 0x00ff;

    return _MODBUS_RTU_PRESET_REQ_LENGTH;
}

/* Builds a RTU response header */
static int _modbus_rtu_build_response_basis(sft_t *sft, uint8_t *rsp)
{
    rsp[0] = sft->slave;
    rsp[1] = sft->function;

    return _MODBUS_RTU_PRESET_RSP_LENGTH;
}

static uint16_t crc16(uint8_t *buffer, uint16_t buffer_length)
{
    uint8_t crc_hi = 0xFF; /* high CRC byte initialized */
    uint8_t crc_lo = 0xFF; /* low CRC byte initialized */
    unsigned int i; /* will index into CRC lookup */

    /* pass through message buffer */
    while (buffer_length--) {
        i = crc_hi ^ *buffer++; /* calculate the CRC  */
        crc_hi = crc_lo ^ table_crc_hi[i];
        crc_lo = table_crc_lo[i];
    }

    return (crc_hi << 8 | crc_lo);
}

int _modbus_rtu_prepare_response_tid(const uint8_t *req, int *req_length)
{
    (*req_length) -= _MODBUS_RTU_CHECKSUM_LENGTH;
    /* No TID */
    return 0;
}

int _modbus_rtu_send_msg_pre(uint8_t *req, int req_length)
{
    uint16_t crc = crc16(req, req_length);
    req[req_length++] = crc >> 8;
    req[req_length++] = crc & 0x00FF;

    return req_length;
}

#ifdef NATIVE_WIN32
/* This simple implementation is sort of a substitute of the select() call, working
 * this way: the win32_ser_select() call tries to read some data from the serial port,
 * setting the timeout as the select() call would. Data read is stored into the
 * receive buffer, that is then consumed by the win32_ser_read() call.
 * So win32_ser_select() does both the event waiting and the reading,
 * while win32_ser_read() only consumes the receive buffer.
 */

static void win32_ser_init(struct win32_ser *ws) {
    /* Clear everything */
    memset(ws,0x00,sizeof(struct win32_ser));
    /* Set file handle to invalid */
    ws->fd = INVALID_HANDLE_VALUE;
}

static int win32_ser_select(struct win32_ser *ws, int max_len, struct timeval *tv) {
    COMMTIMEOUTS comm_to; unsigned int msec = 0;
    /* Check if some data still in the buffer to be consumed */
    if (ws->n_bytes> 0) {
        return 1;
    }
    /* Setup timeouts like select() would do */
    msec = tv->tv_sec * 1000 + tv->tv_usec / 1000;
    if (msec < 1) msec = 1;
    comm_to.ReadIntervalTimeout = msec;
    comm_to.ReadTotalTimeoutMultiplier = 0;
    comm_to.ReadTotalTimeoutConstant = msec;
    comm_to.WriteTotalTimeoutMultiplier = 0;
    comm_to.WriteTotalTimeoutConstant = 1000;
    SetCommTimeouts(ws->fd,&comm_to);
    /* Read some bytes */
    if ((max_len > PY_BUF_SIZE) || (max_len < 0)) {
        max_len = PY_BUF_SIZE;
    }
    if (ReadFile(ws->fd, &ws->buf, max_len, &ws->n_bytes, NULL)) {
        /* Check if some bytes available */
        if (ws->n_bytes > 0) {
            /* Some bytes read */
            return 1;
        } else {
            /* Just timed out */
            return 0;
        }
    } else {
        /* Some kind of error */
        return -1;
    }
}

static int win32_ser_read(struct win32_ser *ws, uint8_t *p_msg, unsigned int max_len) {
    unsigned int n = ws->n_bytes;
    if (max_len < n) {
        n = max_len;
    }
    if (n > 0) {
        memcpy(p_msg,ws->buf,n);
    }
    ws->n_bytes -= n;
    return(n);
}
#endif

ssize_t _modbus_rtu_send(modbus_t *ctx, const uint8_t *req, int req_length)
{
#ifdef NATIVE_WIN32
    modbus_rtu_t *ctx_rtu = ctx->backend_data;
    DWORD n_bytes = 0;
    return (WriteFile(ctx_rtu->w_ser.fd, req, req_length, &n_bytes, NULL)) ? n_bytes : -1;
#else
    return write(ctx->s, req, req_length);
#endif
}

ssize_t _modbus_rtu_recv(modbus_t *ctx, uint8_t *rsp, int rsp_length)
{
#ifdef NATIVE_WIN32
    return win32_ser_read(&((modbus_rtu_t *)ctx->backend_data)->w_ser, rsp, rsp_length);
#else
    return read(ctx->s, rsp, rsp_length);
#endif
}

/* The check_crc16 function shall return the message length if the CRC is
   valid. Otherwise it shall return -1 and set errno to EMBADCRC. */
int _modbus_rtu_check_integrity(modbus_t *ctx, uint8_t *msg,
                                const int msg_length)
{
    uint16_t crc_calculated;
    uint16_t crc_received;

    crc_calculated = crc16(msg, msg_length - 2);
    crc_received = (msg[msg_length - 2] << 8) | msg[msg_length - 1];

    /* Check CRC of msg */
    if (crc_calculated == crc_received) {
        return msg_length;
    } else {
        if (ctx->debug) {
            fprintf(stderr, "ERROR CRC received %0X != CRC calculated %0X\n",
                    crc_received, crc_calculated);
        }
        if (ctx->error_recovery) {
            _modbus_rtu_flush(ctx);
        }
        errno = EMBBADCRC;
        return -1;
    }
}

/* Sets up a serial port for RTU communications */
static int _modbus_rtu_connect(modbus_t *ctx)
{
#ifdef NATIVE_WIN32
    DCB dcb;
#else
    struct termios tios;
    speed_t speed;
#endif
    modbus_rtu_t *ctx_rtu = ctx->backend_data;

    if (ctx->debug) {
        printf("Opening %s at %d bauds (%c, %d, %d)\n",
               ctx_rtu->device, ctx_rtu->baud, ctx_rtu->parity,
               ctx_rtu->data_bit, ctx_rtu->stop_bit);
    }

#ifdef NATIVE_WIN32
    /* Some references here:
     * http://msdn.microsoft.com/en-us/library/aa450602.aspx
     */
    win32_ser_init(&ctx_rtu->w_ser);

    /* ctx_rtu->device should contain a string like "COMxx:" xx being a decimal number */
    ctx_rtu->w_ser.fd = CreateFileA(ctx_rtu->device,
                                        GENERIC_READ | GENERIC_WRITE,
                                        0,
                                        NULL,
                                        OPEN_EXISTING,
                                        0,
                                        NULL);

    /* Error checking */
    if (ctx_rtu->w_ser.fd == INVALID_HANDLE_VALUE) {
        fprintf(stderr, "ERROR Can't open the device %s (%s)\n",
                ctx_rtu->device, strerror(errno));
        return -1;
    }

    /* Save params */
    ctx_rtu->old_dcb.DCBlength = sizeof(DCB);
    if (!GetCommState(ctx_rtu->w_ser.fd, &ctx_rtu->old_dcb)) {
        fprintf(stderr, "ERROR Error getting configuration (LastError %d)\n",
                (int)GetLastError());
        return -1;
    }

    /* Build new configuration (starting from current settings) */
    dcb = ctx_rtu->old_dcb;

    /* Speed setting */
    switch (ctx_rtu->baud) {
        case 110:
            dcb.BaudRate = CBR_110;
            break;
        case 300:
            dcb.BaudRate = CBR_300;
            break;
        case 600:
            dcb.BaudRate = CBR_600;
            break;
        case 1200:
            dcb.BaudRate = CBR_1200;
            break;
        case 2400:
            dcb.BaudRate = CBR_2400;
            break;
        case 4800:
            dcb.BaudRate = CBR_4800;
            break;
        case 9600:
            dcb.BaudRate = CBR_9600;
            break;
        case 19200:
            dcb.BaudRate = CBR_19200;
            break;
        case 38400:
            dcb.BaudRate = CBR_38400;
            break;
        case 57600:
            dcb.BaudRate = CBR_57600;
            break;
        case 115200:
            dcb.BaudRate = CBR_115200;
            break;
        default:
            dcb.BaudRate = CBR_9600;
            printf("WARNING Unknown baud rate %d for %s (B9600 used)\n",
                    ctx_rtu->baud, ctx_rtu->device);
        }

        /* Data bits */
        switch (ctx_rtu->data_bit) {
            case 5:
                dcb.ByteSize = 5;
                break;
            case 6:
                dcb.ByteSize = 6;
                break;
            case 7:
                dcb.ByteSize = 7;
                break;
            case 8:
            default:
                dcb.ByteSize = 8;
                break;
        }

        /* Stop bits */
        if (ctx_rtu->stop_bit == 1)
            dcb.StopBits = ONESTOPBIT;
        else /* 2 */
            dcb.StopBits = TWOSTOPBITS;

        /* Parity */
        if (ctx_rtu->parity == 'N') {
            dcb.Parity = NOPARITY;
            dcb.fParity = FALSE;
        } else if (ctx_rtu->parity == 'E') {
            dcb.Parity = EVENPARITY;
            dcb.fParity = TRUE;
        } else {
            /* odd */
            dcb.Parity = ODDPARITY;
            dcb.fParity = TRUE;
        }

        /* Hardware handshaking left as default settings retrieved */

        /* No software handshaking */
        dcb.fTXContinueOnXoff = TRUE;
        dcb.fOutX = FALSE;
        dcb.fInX = FALSE;

        /* Binary mode (it's the only supported on Windows anyway) */
        dcb.fBinary = TRUE;

        /* Don't want errors to be blocking */
        dcb.fAbortOnError = FALSE;

        /* TODO: any other flags !? */

        /* Setup port */
        if (!SetCommState(ctx_rtu->w_ser.fd, &dcb)) {
            fprintf(stderr, "ERROR Error setting new configuration (LastError %d)\n",
                    (int)GetLastError());
            return -1;
        }
#else
    /* The O_NOCTTY flag tells UNIX that this program doesn't want
       to be the "controlling terminal" for that port. If you
       don't specify this then any input (such as keyboard abort
       signals and so forth) will affect your process

       Timeouts are ignored in canonical input mode or when the
       NDELAY option is set on the file via open or fcntl */
    ctx->s = open(ctx_rtu->device, O_RDWR | O_NOCTTY | O_NDELAY | O_EXCL);
    if (ctx->s == -1) {
        fprintf(stderr, "ERROR Can't open the device %s (%s)\n",
                ctx_rtu->device, strerror(errno));
        return -1;
    }

    /* Save */
    tcgetattr(ctx->s, &(ctx_rtu->old_tios));

    memset(&tios, 0, sizeof(struct termios));

    /* C_ISPEED     Input baud (new interface)
       C_OSPEED     Output baud (new interface)
    */
    switch (ctx_rtu->baud) {
    case 110:
        speed = B110;
        break;
    case 300:
        speed = B300;
        break;
    case 600:
        speed = B600;
        break;
    case 1200:
        speed = B1200;
        break;
    case 2400:
        speed = B2400;
        break;
    case 4800:
        speed = B4800;
        break;
    case 9600:
        speed = B9600;
        break;
    case 19200:
        speed = B19200;
        break;
    case 38400:
        speed = B38400;
        break;
    case 57600:
        speed = B57600;
        break;
    case 115200:
        speed = B115200;
        break;
    default:
        speed = B9600;
        if (ctx->debug) {
            fprintf(stderr,
                    "WARNING Unknown baud rate %d for %s (B9600 used)\n",
                    ctx_rtu->baud, ctx_rtu->device);
        }
    }

    /* Set the baud rate */
    if ((cfsetispeed(&tios, speed) < 0) ||
        (cfsetospeed(&tios, speed) < 0)) {
        return -1;
    }

    /* C_CFLAG      Control options
       CLOCAL       Local line - do not change "owner" of port
       CREAD        Enable receiver
    */
    tios.c_cflag |= (CREAD | CLOCAL);
    /* CSIZE, HUPCL, CRTSCTS (hardware flow control) */

    /* Set data bits (5, 6, 7, 8 bits)
       CSIZE        Bit mask for data bits
    */
    tios.c_cflag &= ~CSIZE;
    switch (ctx_rtu->data_bit) {
    case 5:
        tios.c_cflag |= CS5;
        break;
    case 6:
        tios.c_cflag |= CS6;
        break;
    case 7:
        tios.c_cflag |= CS7;
        break;
    case 8:
    default:
        tios.c_cflag |= CS8;
        break;
    }

    /* Stop bit (1 or 2) */
    if (ctx_rtu->stop_bit == 1)
        tios.c_cflag &=~ CSTOPB;
    else /* 2 */
        tios.c_cflag |= CSTOPB;

    /* PARENB       Enable parity bit
       PARODD       Use odd parity instead of even */
    if (ctx_rtu->parity == 'N') {
        /* None */
        tios.c_cflag &=~ PARENB;
    } else if (ctx_rtu->parity == 'E') {
        /* Even */
        tios.c_cflag |= PARENB;
        tios.c_cflag &=~ PARODD;
    } else {
        /* Odd */
        tios.c_cflag |= PARENB;
        tios.c_cflag |= PARODD;
    }

    /* Read the man page of termios if you need more information. */

    /* This field isn't used on POSIX systems
       tios.c_line = 0;
    */

    /* C_LFLAG      Line options

       ISIG Enable SIGINTR, SIGSUSP, SIGDSUSP, and SIGQUIT signals
       ICANON       Enable canonical input (else raw)
       XCASE        Map uppercase \lowercase (obsolete)
       ECHO Enable echoing of input characters
       ECHOE        Echo erase character as BS-SP-BS
       ECHOK        Echo NL after kill character
       ECHONL       Echo NL
       NOFLSH       Disable flushing of input buffers after
       interrupt or quit characters
       IEXTEN       Enable extended functions
       ECHOCTL      Echo control characters as ^char and delete as ~?
       ECHOPRT      Echo erased character as character erased
       ECHOKE       BS-SP-BS entire line on line kill
       FLUSHO       Output being flushed
       PENDIN       Retype pending input at next read or input char
       TOSTOP       Send SIGTTOU for background output

       Canonical input is line-oriented. Input characters are put
       into a buffer which can be edited interactively by the user
       until a CR (carriage return) or LF (line feed) character is
       received.

       Raw input is unprocessed. Input characters are passed
       through exactly as they are received, when they are
       received. Generally you'll deselect the ICANON, ECHO,
       ECHOE, and ISIG options when using raw input
    */

    /* Raw input */
    tios.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);

    /* C_IFLAG      Input options

       Constant     Description
       INPCK        Enable parity check
       IGNPAR       Ignore parity errors
       PARMRK       Mark parity errors
       ISTRIP       Strip parity bits
       IXON Enable software flow control (outgoing)
       IXOFF        Enable software flow control (incoming)
       IXANY        Allow any character to start flow again
       IGNBRK       Ignore break condition
       BRKINT       Send a SIGINT when a break condition is detected
       INLCR        Map NL to CR
       IGNCR        Ignore CR
       ICRNL        Map CR to NL
       IUCLC        Map uppercase to lowercase
       IMAXBEL      Echo BEL on input line too long
    */
    if (ctx_rtu->parity == 'N') {
        /* None */
        tios.c_iflag &= ~INPCK;
    } else {
        tios.c_iflag |= INPCK;
    }

    /* Software flow control is disabled */
    tios.c_iflag &= ~(IXON | IXOFF | IXANY);

    /* C_OFLAG      Output options
       OPOST        Postprocess output (not set = raw output)
       ONLCR        Map NL to CR-NL

       ONCLR ant others needs OPOST to be enabled
    */

    /* Raw ouput */
    tios.c_oflag &=~ OPOST;

    /* C_CC         Control characters
       VMIN         Minimum number of characters to read
       VTIME        Time to wait for data (tenths of seconds)

       UNIX serial interface drivers provide the ability to
       specify character and packet timeouts. Two elements of the
       c_cc array are used for timeouts: VMIN and VTIME. Timeouts
       are ignored in canonical input mode or when the NDELAY
       option is set on the file via open or fcntl.

       VMIN specifies the minimum number of characters to read. If
       it is set to 0, then the VTIME value specifies the time to
       wait for every character read. Note that this does not mean
       that a read call for N bytes will wait for N characters to
       come in. Rather, the timeout will apply to the first
       character and the read call will return the number of
       characters immediately available (up to the number you
       request).

       If VMIN is non-zero, VTIME specifies the time to wait for
       the first character read. If a character is read within the
       time given, any read will block (wait) until all VMIN
       characters are read. That is, once the first character is
       read, the serial interface driver expects to receive an
       entire packet of characters (VMIN bytes total). If no
       character is read within the time allowed, then the call to
       read returns 0. This method allows you to tell the serial
       driver you need exactly N bytes and any read call will
       return 0 or N bytes. However, the timeout only applies to
       the first character read, so if for some reason the driver
       misses one character inside the N byte packet then the read
       call could block forever waiting for additional input
       characters.

       VTIME specifies the amount of time to wait for incoming
       characters in tenths of seconds. If VTIME is set to 0 (the
       default), reads will block (wait) indefinitely unless the
       NDELAY option is set on the port with open or fcntl.
    */
    /* Unused because we use open with the NDELAY option */
    tios.c_cc[VMIN] = 0;
    tios.c_cc[VTIME] = 0;

    if (tcsetattr(ctx->s, TCSANOW, &tios) < 0) {
        return -1;
    }
#endif

    return 0;
}

void _modbus_rtu_close(modbus_t *ctx)
{
    /* Closes the file descriptor in RTU mode */
    modbus_rtu_t *ctx_rtu = ctx->backend_data;

#ifdef NATIVE_WIN32
    /* Revert settings */
    if (!SetCommState(ctx_rtu->w_ser.fd, &ctx_rtu->old_dcb))
        fprintf(stderr, "ERROR Couldn't revert to configuration (LastError %d)\n",
                (int)GetLastError());

    if (!CloseHandle(ctx_rtu->w_ser.fd))
        fprintf(stderr, "ERROR Error while closing handle (LastError %d)\n",
                (int)GetLastError());
#else
    tcsetattr(ctx->s, TCSANOW, &(ctx_rtu->old_tios));
    close(ctx->s);
#endif
}

int _modbus_rtu_flush(modbus_t *ctx)
{
#ifdef NATIVE_WIN32
    modbus_rtu_t *ctx_rtu = ctx->backend_data;
    ctx_rtu->w_ser.n_bytes = 0;
    return ( FlushFileBuffers(ctx_rtu->w_ser.fd) == FALSE );
#else
    return tcflush(ctx->s, TCIOFLUSH);
#endif
}

int _modbus_rtu_listen(modbus_t *ctx, int nb_connection)
{
    if (ctx->debug) {
        fprintf(stderr, "Not implemented");
    }

    errno = EINVAL;
    return -1;
}

int _modbus_rtu_accept(modbus_t *ctx, int *socket)
{
    if (ctx->debug) {
        fprintf(stderr, "Not implemented");
    }

    errno = EINVAL;
    return -1;
}

int _modbus_rtu_select(modbus_t *ctx, fd_set *rfds, struct timeval *tv, int msg_length_computed, int msg_length)
{
    int s_rc;
#ifdef NATIVE_WIN32
    s_rc = win32_ser_select(&(((modbus_rtu_t*)ctx->backend_data)->w_ser), msg_length_computed, tv);
    if (s_rc == 0) {
        errno = ETIMEDOUT;
        return -1;
    }

    if (s_rc < 0) {
        _error_print(ctx, "select");
        if (ctx->error_recovery && (errno == EBADF)) {
            modbus_close(ctx);
            modbus_connect(ctx);
            errno = EBADF;
            return -1;
        } else {
            return -1;
        }
    }
#else
    while ((s_rc = select(ctx->s+1, rfds, NULL, NULL, tv)) == -1) {
        if (errno == EINTR) {
            if (ctx->debug) {
                fprintf(stderr, "A non blocked signal was caught\n");
            }
            /* Necessary after an error */
            FD_ZERO(rfds);
            FD_SET(ctx->s, rfds);
        } else {
            _error_print(ctx, "select");
            if (ctx->error_recovery && (errno == EBADF)) {
                modbus_close(ctx);
                modbus_connect(ctx);
                errno = EBADF;
                return -1;
            } else {
                return -1;
            }
        }
    }

    if (s_rc == 0) {
        /* Timeout */
        if (msg_length == (ctx->backend->header_length + 2 +
                           ctx->backend->checksum_length)) {
            /* Optimization allowed because exception response is
               the smallest trame in modbus protocol (3) so always
               raise a timeout error.
               Temporary error before exception analyze. */
            errno = EMBUNKEXC;
        } else {
            errno = ETIMEDOUT;
            _error_print(ctx, "select");
        }
        return -1;
    }
#endif

    return s_rc;
}

int _modbus_rtu_filter_request(modbus_t *ctx, int slave)
{
    /* Filter on the Modbus unit identifier (slave) in RTU mode */
    if (slave != ctx->slave && slave != MODBUS_BROADCAST_ADDRESS) {
        /* Ignores the request (not for me) */
        if (ctx->debug) {
            printf("Request for slave %d ignored (not %d)\n",
                   slave, ctx->slave);
        }
        return 1;
    } else {
        return 0;
    }
}

const modbus_backend_t _modbus_rtu_backend = {
    _MODBUS_BACKEND_TYPE_RTU,
    _MODBUS_RTU_HEADER_LENGTH,
    _MODBUS_RTU_CHECKSUM_LENGTH,
    MODBUS_RTU_MAX_ADU_LENGTH,
    _modbus_set_slave,
    _modbus_rtu_build_request_basis,
    _modbus_rtu_build_response_basis,
    _modbus_rtu_prepare_response_tid,
    _modbus_rtu_send_msg_pre,
    _modbus_rtu_send,
    _modbus_rtu_recv,
    _modbus_rtu_check_integrity,
    _modbus_rtu_connect,
    _modbus_rtu_close,
    _modbus_rtu_flush,
    _modbus_rtu_listen,
    _modbus_rtu_accept,
    _modbus_rtu_select,
    _modbus_rtu_filter_request
};

/* Allocate and initialize the modbus_t structure for RTU
   - device: "/dev/ttyS0"
   - baud:   9600, 19200, 57600, 115200, etc
   - parity: 'N' stands for None, 'E' for Even and 'O' for odd
   - data_bits: 5, 6, 7, 8
   - stop_bits: 1, 2
   - slave: slave number of the caller
*/
modbus_t* modbus_new_rtu(const char *device,
                         int baud, char parity, int data_bit,
                         int stop_bit, int slave)
{
    modbus_t *ctx;
    modbus_rtu_t *ctx_rtu;

    ctx = (modbus_t *) malloc(sizeof(modbus_t));
    _modbus_init_common(ctx);
    if (_modbus_set_slave(ctx, slave) == -1) {
        return NULL;
    }

    ctx->backend = &_modbus_rtu_backend;

    ctx->backend_data = (modbus_rtu_t *) malloc(sizeof(modbus_rtu_t));
    ctx_rtu = (modbus_rtu_t *)ctx->backend_data;
#if defined(OpenBSD)
    strlcpy(ctx_rtu->device, device, sizeof(ctx_rtu->device));
#else
    strcpy(ctx_rtu->device, device);
#endif
    ctx_rtu->baud = baud;
    if (parity == 'N' || parity == 'E' || parity == 'O') {
        ctx_rtu->parity = parity;
    } else {
        errno = EINVAL;
        return NULL;
    }
    ctx_rtu->data_bit = data_bit;
    ctx_rtu->stop_bit = stop_bit;

    return ctx;
}